More information is coming out about the research done by the Human Genome Project into our genetic history. Humans have 23 pairs of chromosomes with about 30,000 genes made up of DNA strands in a specific order. That order can change over time through mutation and mutations occur over time in a fairly constant rate. Because of that constant rate, genetic mutations can be used to measure when species became separate.
When comparing the genes of chimps and humans it was discovered that both species shared a common ancestor who supplied both their X chromosomes and that this occurred more recently than ancestors who supplied their other chromosomes. Their theory is that human and chimp species broke apart from each other on two different occasions. This leads to the assumption that humans and chimps actually may have interbred with each other for several millennia. This slower process of species differentiation was favored by Darwin, but had fallen out of favor more recently with scientists who believed that hybrid offspring, being infertile, would cause a more sudden break in species, It would appear now that Darwin may have been right again.